1. Field of the Invention:
The present invention relates to electro-optic sensor systems. More particularly, the present invention relates to ultra-wide field of view scanning electro-optic sensor systems.
2. Description of the Related Art
A typical acquisition sensor requires an optical system capable of very wide field of view. Most conventional wide field of view scanning systems are either of the fisheye lens form or concentric Bouwers system. Unfortunately, most wide field of view systems such as a fisheye lens or Bouwers system are generally large.
For a fisheye lens, the overall length from the vertex of the lens to the focal plane is typically at least 6 times the effective focal length (EFL). Consequently, these lenses are large and heavy. In many cases, aspheric lenses are needed for both aberration and distortion correction. Additionally, the implementation of a scanner near the pupil plane (aperture stop), necessitated by this design, is a challenging task.
A Bouwers system is a catadioptric monocentric optical system consisted of a refractive corrector and a spherical mirror. Although the Bouwers system is a concentric design form, the overall length is still 2 times the EFL. Additionally, the generic central obscuration characteristics associated with a Bouwers system often limits the effective F/# and the field of view coverage. Although a scanner close to the aperture stop can be decentered to alleviate the obscuration problem, the F/# and field of view are further compromised.
Accordingly, a need has existed in the art for a compact, lightweight wide field of view optical system. The need is addressed somewhat by U.S. patent application Ser. No. 09/413,985, entitled Ultra-Wide Field of View Concentric Sensor System, filed May 19, 1999 by C. W. Chen et al. and U.S. patent application Ser. No. 09/413,688, entitled Ultra-Wide Field of View Concentric Scanning Sensor System, filed Oct. 7, 1999 by C. W. Chen et al., the teachings of which are incorporated herein by reference. In the referenced Applications, Chen et al. disclose and claim a compact, high numerical aperture, high resolution, ultra-wide filed-of-view concentric optical sensor. However, the required curved focal surface array (FSA) concentric to the center of the optical system adds cost and technical complexity in fabrication.
Hence, a need remains in the art for a compact, lightweight ultra-wide field of view optical system suitable for use in acquisition applications that is easy to manufacture.
The need in the art is addressed by the optical system of the present invention. In a most general embodiment, the inventive optical arrangement includes an at least semi-spherical lens having a base; a reflective surface centered at a center of said base and parallel thereto; and an array of detectors dispose to receive electromagnetic energy received through said lens and reflected by said surface.
In the illustrative embodiment, the semi-spherical lens is a ball lens. A dome lens is disposed over the ball lens and concentric therewith. Electromagnetic energy is received through the dome and ball lenses and reflected by the mirror to the detector array. The reflective surface of the mirror is aspheric and designed to effect aberration correction. A mechanism is included for rotating the ball lens thereby causing the system to scan. A field lens assembly (FSA) is disposed between the detectors and the mirrors. The field lens assembly allows for the use of flat piece-wise sensor chip assemblies (SCAs). An array of SCAs is employed to provide a focal surface array. In a first alternative embodiment, the focal surface array is a 120xc2x0 arc around the ball lens. In a second alternative embodiment, the mirror is reflective on both sides thereof to double the scanning rate. In the preferred implementation, the ball lens is cut in half or provided as two separate halves. Each side of the mirror is etched or cut into a half of the ball lens and the two halves are joined to provide a complete sphere. In yet another alternative embodiment, an external multi-faceted reflective surface is scanned to direct energy into the ball lens thereby eliminating the requirement to rotate the ball lens.
In short, the invention is a compact, high numerical aperture, high resolution, ultra-wide field of view (FOV) concentric scanning optical sensor. The field of view coverage can be as large as 160 degrees. Yet, the sensor volume is approximately 20 times smaller than that of the corresponding fisheye lens. The use of flat SCAs allows the system to have the compact advantages of a concentric system without the need of expensive curved FSAs.